/**
 *    Copyright 2009-2015 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.scripting.xmltags;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import ognl.Ognl;
import ognl.OgnlException;

import org.apache.ibatis.builder.BuilderException;

/**
 * Caches OGNL parsed expressions.
 * 
 * @see http://code.google.com/p/mybatis/issues/detail?id=342
 *      OGNL缓存,根据以上链接，大致是说ognl有性能问题，所以加了一个缓存
 *
 * @author Eduardo Macarron
 */
public final class OgnlCache {
	// 表达式缓存
	private static final Map<String, Object> expressionCache = new ConcurrentHashMap<String, Object>();

	private OgnlCache() {
		// Prevent Instantiation of Static Class
	}

	public static Object getValue(String expression, Object root) {
		try {
			//创建并返回一个OGNL表达式评估新标准的命名上下文
			Map<Object, OgnlClassResolver> context = Ognl.createDefaultContext(root, new OgnlClassResolver());
			return Ognl.getValue(parseExpression(expression), context, root);
		} catch (OgnlException e) {
			throw new BuilderException("Error evaluating expression '" + expression + "'. Cause: " + e, e);
		}
	}

	/**
	 * 解析表达式
	 * 
	 * @param expression
	 * @return
	 * @throws OgnlException
	 */
	private static Object parseExpression(String expression) throws OgnlException {
		// 得到节点
		Object node = expressionCache.get(expression);
		if (node == null) {
			// 大致意思就是OgnlParser.topLevelExpression很慢，所以加个缓存，放到ConcurrentHashMap里面
			node = Ognl.parseExpression(expression);
			expressionCache.put(expression, node);
		}
		return node;
	}

}
